Role of PDK1 in pancreatic cancer cell fitness under tumor nutrient stress

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PROJECT SUMMARY Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive cancer type in urgent need of more effective treatment options. A notable feature of PDAC tumors is the highly altered nutrient conditions present within the tumor microenvironment (TME) caused by poor perfusion from the supporting tumor vasculature. Although PDAC cells can rapidly proliferate within these suboptimal nutrient conditions, the metabolic adaptations they rely upon to do so remain unknown. Furthermore, understanding these adaptations has the potential to reveal therapeutically targetable vulnerabilities of PDAC cells in vivo. Towards this end, we have developed a novel medium formulation (TIFM) that recapitulates the nutrient conditions present within PDAC tumors in vivo, in order to study the metabolic responses of PDAC cells to tumor nutrient stress using tractable ex vivo models. Applying pooled CRISPR-interference (CRISPRi) screening to this new model system, we surprisingly identified a hypoxia-induced kinase, pyruvate-dehydrogenase kinase 1 (PDK1), as being critical to the fitness of PDAC cells in TIFM, even under normoxia conditions. PDK1 is a kinase normally activated by hypoxia to inhibit the pyruvate dehydrogenase (PDH) complex and thus redirect pyruvate-derived carbon away from the TCA cycle and towards lactic acid fermentation. The overall goal of this proposal is to understand how and why PDK1 and aerobic glycolysis become critical to PDAC cellular fitness during tumor nutrient stress, and to evaluate in vivo the functional dependence of PDAC cells on suppressed pyruvate oxidation for tumor growth. I hypothesize that two potential mechanisms may underlie PDK1 dependency in TIFM: (1) PDK1 promotes aerobic glycolysis to prevent the production of cytotoxic levels of reactive oxygen species (ROS) or (2) supports NAD+/NADH cofactor balance for macromolecule biosynthesis. Further, I hypothesize that PDK1 activity is upregulated under TIFM conditions through a nutrient-sensitive mTORC2-AKT-PDK1 signaling axis. We will investigate this hypothesis in three specific aims. Aim 1. Functional and metabolic assays will be performed to determine the adaptive function of PDK1 under tumor nutrient stress. Aim 2. Functional and biochemical assays will be performed to identify the nutrient factor in TIFM responsible for PDK1 dependency and elucidate the signaling pathway that communicates its availability to the TCA cycle. Aim 3. Animal studies will be performed to determine the essentiality of suppressed pyruvate oxidation for PDAC tumor growth in vivo. These findings reveal a critical metabolic dependency of cancer cells in adaptation to the nutrient stress present in the TME. Furthermore, understanding this adaptation may reveal novel therapeutic strategies for managing pancreatic cancer in the clinic based on metabolic constraints set by the tumor microenvironment.